Verify the legacy account before accepting registration
Imported issue was from daniel:
XEP-0100 asks that the username and password of the legacy account actually be verified. Basically it boils down to accepting username and password, logging in, if it works, holding the session "in the background" with the expectation that this user will go ahead and log in immediately after registration is complete.
This is actually completed! I wasn't aware I had a previous issue about it.